(RNS) — American Atheists, one of the most prominent of the half-dozen national freethought activist organizations, has placed its highly visible president, David Silverman, on leave after an unspecified complaint.

Nick Fish, national program director for American Atheists, who has taken over daily operations while Silverman is on leave, said in an email that the complaint was in relation to “alleged violations of the American Atheists code of conduct and staff handbook.”

On Tuesday (April 10), the New Jersey-based organization issued a statement about the move.

“The Board takes very seriously the concerns expressed and, in accordance with organization policies, the Board has placed Mr. Silverman on leave while an independent investigation is conducted,” the statement reads. “Mr. Silverman has pledged his full cooperation with the investigation.”

The statement says board members received “a complaint” on Saturday about Silverman. It did not describe the nature of the complaint. Silverman is receiving his salary while on leave, Fish said.

Hemant Mehta, a popular atheist speaker, announced the sidelining of Silverman on his blog, Friendly Atheist.

“On a personal note, this is disturbing news in part because we don’t know the specifics of the allegations (and critics will inevitably use that to their advantage) and in part because I’ve known Dave in a professional context for about 15 years and think the world of him,” he said on the blog. “I sincerely hope the investigation comes up empty.”

Silverman, 51, has been with American Atheists since 2004 and became president of the 6,000-member group in 2010. He is famous beyond atheist circles for his multiple appearances on Fox News to debate the “War on Christmas” and the removal of religious symbols from public property.

The group made national headlines under his leadership when it placed billboards denouncing Christmas as a religious holiday in New York City’s Times Square.